This blog provides updated forecasts and comments on current weather or other topics

Wednesday, November 14, 2012

Heavy Rains Return to the Northwest

The last two weeks of November are climatologically the wettest period of the entire year.

This year won't disappoint.

Let me begin by showing you a neat chart never before seen on this blog. The TOTAL precipitation forecast for the next 8 days from the National Weather Service GFS model. This model has relatively modest resolution, so it is not capturing the full amounts over our terrain, but you will get the picture.

Very wet from coastal BC down to northern CA (5-10 inches is commonplace, with northwest CA "enjoying" 10-15 inches. No rain in Arizona and New Mexico and much of the NE U.S.

Lets take a look what the higher resolution UW WRF model is giving. Let me first show you the first 72 hr (ending 4 PM on Saturday). Modest precipitation over western Washington, but the heaviest rain (2-5 inches) over the mountains of NW CA.

But the next 72 hr really opens the skies, particularly over NW CA and western Oregon. Some locations near the CA/OR border are forecast to receive over ten inches of rain. We are talking about real flood potential. Notice that even eastern WA is getting wet.

Why so much rain? A large, cold trough will form over the eastern Pacific, establishing a persistent flow of moist air from the west and southwest (see upper level map for next Monday at 1 PM):

Such heavy precipitation events are inevitably associated with atmospheric rivers--relatively narrow currents of enhanced water vapor. For this case, the atmospheric river will be coming from the west, not from Hawaii, so don't call it a pineapple express (see graphic).

Teriyaki express? Will have to work on a good name....

What about snow in the mountains?

Temperatures will be marginal in the lower passes (Snoqualmie/Stevens), so there should be a mixture of rain and snow there---I doubt they will have enough to open for Thanksgiving. But higher ski areas (Crystal, Baker) should get substantial amounts (foot plus) and may be able to open by Turkey Day. Keep tuned.